This invention relates to tilting pad thrust bearing assemblies and more particularly to a unique arrangement of equalizing links for use in a tilting pad thrust bearing assembly or other device.
Equalizing links have been used in tilting pad thrust bearings for many years. In order to reduce friction effects, various shapes and arrangements of the equalizing links used in tilting pad thrust bearings have been proposed. See, for example, U.S. Pat. Nos. 1,361,073; 1,666,521; 3,655,250 and 4,403,873, all showing different arrangements of equalizing links and tilting pads. However, these designs either do not equalize at all under load, cannot be used with the type of thrust pads currently in use, or they are uneconomical to manufacture.
Based upon my more than 40 years experience in the development and manufacture of tilting pad thrust bearing assemblies, I have invented an essentially friction free tilting pad assembly for use in thrust bearings and other devices.
In tilting pad thrust bearings such as those shown in the foregoing patents, the thrust load along the axis of the rotating shaft is transmitted from a collar affixed to the shaft to a plurality of individual tilting thrust pads arranged in a circle, then to a plurality of upper equalizing links, then to a plurality of lower equalizing links, and finally to the stationary machine housing.
The thrust collar mounted on the rotating shaft has a planar surface which intersects the axis of the rotating shaft at exactly 90.degree.. Since the individual tilting pads are independent of each other, the loads carried by each pad will vary according to its distance from the planar surface of the rotating collar. Variations in the distance between the planar surface of the collar and the individual pads can result from manufacturing tolerances on the individual component parts or from machine housing mounting surfaces for the retaining ring being out of square with the axis of the shaft. The purpose of the equalizing links is to promptly and automatically move the several thrust pads into positions equidistant from the rotating planar surface of the shaft's collar resulting in equal thrust pad loads.
Tilting pad thrust bearings depend upon hydrodynamic action to produce the fluid film pressure which supports the rotating thrust load and the capacity of the fluid film bearing will be greatest when a uniform distance is maintained between each of the individual thrust pads and the planar surface of the rotating collar.
Most state of the art equalizing links and those shown in the aforementioned patents do not promptly equalize under load because of the friction generated by the upper and lower links sliding across each other seeking to adjust due to changes requiring prompt movement of the upper equalizing links. These changes include changes caused by thermal expansion or contraction of the machine housing during operation and changes due to misalignment of one or more of the bearing components.
Simply stated, my invention eliminates or reduces to a bare minimum the sliding friction generated by prior tilting pad arrangements. By my invention the contact between the adjacent surfaces of links is a rolling contact rather than a sliding contact.
The superiority of this invention over state of the art designs is accomplished by a combination of the following design features:
1. The contact points between adjacent upper and lower equalizing links and the pivot axes of those adjacent links lie in a common plane when the equalizing links are in a position of zero pivoting angle.
2. The lower links are supported on horizontally mounted cylindrical pins.
3. The contact points between adjacent thrust pads and upper links are spherical surfaces of large radius.
In addition to the foregoing design features providing a rolling and hence essentially friction free movement of the equalizing links and the tilting pads, the arrangement provides low bearing pressures with the tilting pad retaining ring.
My invention can be used with a retaining ring whose inner boundry is either an equilateral polygon or circular in plan view, although it has the most efficient equalizing movement in its polygonal form. However, there will be manufacturing economies in using the more common circular form.
The thrust pad load is transmitted to the upper equalizing link through the thrust pad support. The upper equalizing link transmits its load to the lower equalizing link by interacting contact points between the links. These contact points are located to provide equal lever arms from the pivot points of the equalizing links. The surface of the lower link for interacting contact is in a plane containing the axis of the pivoting point of the lower equalizing link. The interacting contact surface of the upper link is a slightly crowned cylinder whose axis is radial and bisects the angle between the radial pivoting axis of adjacent upper and lower equalizing links. The axis of this crowned cylinder is in a plane parallel to the pivoting axis of the upper link. The axis of pivoting of the upper equalizing link is the result of the shape and fit of the link in the track of the retaining ring and the tangential retaining pin.
The lower equalizing link transmits its load into the retaining ring by a cylindrical member which is also the pivot point of the lower link. This cylindrical member can be stub shafts integral with the lower link and having a 180.degree. bearing surface in the retaining ring. The cylindrical member can alternatively be a fixed round pin in the retaining ring extending through a hole in the lower link. In either design the load transmitting member is adequately proportioned to produce low bearing pressures.
The upper equalizing link is retained tangentially by a ball member permitting the necessary rise and fall of the upper equalizing link to produce equalization of the thrust pad loads as required. A radial pin extending inward from the outer rim of the retaining ring and engaging a vertical slot in the upper equalizing link would provide the same design requirements. In either design the center of the ball or pin must be in the same plane as the pivoting point of the lower equalizing link and the interacting contact points in the position of zero pivoting angle of all links.
In this design the vertical sides of the equalizing links can be parallel planar surfaces or concentric cylindrical surfaces. The design using parallel planar surfaces allows greater link pivoting movement with closer clearances with the retaining ring walls with resulting greater equalization movement for uses other than thrust bearings. The cylindrical design allows sufficient pivoting movement for the relatively small equalizing movement required for most tilting pad thrust bearings.
The compact unique design of my equalizing links provides sufficient space in the retaining ring to permit screws to secure a separate piece to be used as a thrust pad separator. The thrust pad separator can be varied to permit a wide range of lubrication systems such as flood, spray, and directed lubrication. The separator can also be varied to permit offset thrust pad supports as well as the common center thrust pad support.
The design features of my invention will meet the space requirements which have been accepted by the major equalizing tilting pad thrust bearing manufacturers. All components are sized to carry accepted industry rated loads with ample factors of safety using common heat treated materials.
I have constructed a prototype of the titling pads and equalizing links made in accordance with my invention and preliminary tests indicate that the assembly operates essentially friction free with rolling contact between adjacent upper and lower equalizing links.